Recent years have seen increasing high performance in electronic devices such as mobile information devices and information appliances following the development of digital technology. With the increased high performance in these electronic devices, miniaturization and increase in speed of semiconductor memory devices used are rapidly advancing. Among these, application for large-capacity nonvolatile memories represented by a flash memory is rapidly expanding. In addition, as a next-generation new-type nonvolatile memory to replace the flash memory, research and development on a variable resistance nonvolatile storage element which uses what is called a variable resistance element is advancing. Here, variable resistance element refers to an element which has a property in which a resistance value reversibly changes according to electrical signals, and is capable of storing information corresponding to the resistance value in a nonvolatile manner.
As an example of such variable resistance element, there is proposed a nonvolatile memory element having a variable resistance layer in which transition metal oxides of different oxygen content atomic percentages are stacked. For example, Patent Literature 1 discloses selectively causing the occurrence of oxidation/reduction reaction in an electrode interface which is in contact with a variable resistance layer having a high oxygen content atomic percentage, to stabilize resistance change.
The aforementioned conventional variable resistance element includes a lower electrode, a variable resistance layer, and an upper electrode, and a memory array is configured from a two-dimensional or three-dimensional array of such variable resistance element. In each of the variable resistance elements, the variable resistance layer is of a stacked structure including a first variable resistance layer and a second variable resistance layer, and the first and second variable resistance layers comprise the same type of transitional metal oxide. The oxygen content atomic percentage of the transitional metal oxide comprised in the second variable resistance layer is higher than the oxygen content atomic percentage of the transitional metal oxide comprised in the first variable resistance layer. By adopting such a structure, when voltage is applied to the variable resistance element, most of the voltage is applied to the second variable resistance layer which has a high oxygen content atomic percentage and exhibits a higher resistance value. Furthermore, oxygen which can contribute to reaction is abundant in the vicinity of the interface between the upper electrode and the second variable resistance layer. Therefore, oxidation/reduction reaction occurs selectively at the interface between the upper electrode and the second variable resistance layer, and stable resistance change can be realized.
With respect to the conventional nonvolatile memory element, the inventors of the present invention have developed a variable resistance nonvolatile memory element and a method of manufacturing the same, capable of lowering the break voltage initially applied to the variable resistance element in order to transition to a state in which resistance change is started, and suppressing variation of break voltage among the elements, as shown in Patent Literature 2.
FIG. 15 (a) is a cross-sectional view (cross-sectional view at A-A′ in FIG. 15 (b)) of an exemplary configuration of a conventional first nonvolatile memory element 40, and FIG. 15 (b) is a plan view of a first variable resistance layer 106a in FIG. 15 (a). It should be noted that, hereinafter, a cross-sectional view refers to a diagram of the inside of a plane including a line parallel to the stacking direction of a variable resistance element, and a plan view refers to a view seen from the stacking direction of the variable resistance element.
A first nonvolatile memory element 40 includes a substrate 100 on which a first line 101 is formed, a first interlayer insulating layer 102 formed above the substrate 100, and a first contact plug 104 formed inside a first contact hole 103 which penetrates through the first interlayer insulating layer 102 and reaches the first line 101. In addition, a variable resistance element including a lower electrode 105, a variable resistance layer 106, and an upper electrode 107 is formed covering the first contact plug 104. A second interlayer insulating layer 108 is formed covering the variable resistance element, a second contact hole is formed penetrating through the second interlayer insulating layer 108 and reaching the upper electrode 107, and a second contact plug 110 is formed inside the second contact hole 109. A second line 111 is formed covering the second contact plug 110.
Here, the variable resistance layer 106 is configured of the stacked structure of a first variable resistance layer 106a and a second variable resistance layer 106b. The first variable resistance layer 106a comprises a transitional metal oxide having an oxygen-deficient tantalum oxide (TaOx, 0<x<2.5) as a primary component. The oxygen content atomic percentage of a second transitional metal oxide forming the second variable resistance layer 106b is higher than the oxygen content atomic percentage of the first transitional metal oxide forming the first variable resistance layer 106a. For example, assuming that the second variable resistance layer 106b comprises a tantalum oxide (TaOy), then x<y. When the second variable resistance layer 106b comprises a transitional metal oxide other than tantalum, the second variable resistance layer 106b comprises a material having less degree of oxygen deficiency from the stoichiometric composition exhibiting insulating properties.
A straight step 106ax (height: 1 to 30 nm, length 500 nm) such as that shown in FIG. 15 (b) is formed in the surface (the interface with the second variable resistance layer 106b) of the first variable resistance layer 106a, and a straight bend (i.e., stepped portion) 106bx is created in a surface, above the step 106ax, of the second variable resistance layer 106b formed above the first variable resistance layer 106a. 
FIG. 16 (a) is a cross-sectional view (cross-sectional view at A-A′ in FIG. 16 (b)) of an exemplary configuration of a conventional second nonvolatile memory element 50, and FIG. 16 (b) is a plan view of the first variable resistance layer 106a in FIG. 16 (a).
The difference between the second nonvolatile memory element 50 and the first nonvolatile memory element 40 is in the shape of the step formed in the first variable resistance layer 106a. In the first nonvolatile memory element 40, the step 106ax formed in the surface of the first variable resistance layer 106a is straight, whereas, in the second nonvolatile memory element 50, a ring-shaped step 106ay is formed. Accordingly, a bend i.e., stepped portion) 106by of the second variable resistance layer 106b is also ring-shaped.
According to the configurations in FIG. 15 and FIG. 16, the bend 106bx or 106by of the second variable resistance layer 106b is formed above the step 106ax or 106ay of the first variable resistance layer 106a, and thus it is possible to cause the initial break phenomenon even with a low voltage, with the bend 106bx or 106by as a starting point. Furthermore, since the step shape is formed in an intentional and controlled manner, the shape of the bend of the second variable resistance layer 106b is stable, and thus variation in break voltage does not increase. With this, lowering the initial break voltage and suppressing variation therein can both be achieved.
FIG. 17A (a) is a cross-sectional view (cross-sectional view at A-A′ in FIG. 17A (b)) of an exemplary configuration of a conventional third nonvolatile memory element 60, FIG. 17A (b) is a plan view of the first variable resistance layer 106a in FIG. 17A (a), and FIG. 17B is a perspective view of the first variable resistance layer 106a in FIG. 17A.
The difference between the third nonvolatile memory element 60 and the first nonvolatile memory element 40 is in the shape of the step formed in the first variable resistance layer 106a. Specifically, in the first nonvolatile memory element 40, the step 106ax formed in the surface of the first variable resistance layer 106a is a single straight step, whereas, in the third nonvolatile memory element 60, the two linear steps of steps 106ax1 and 106ax2 are formed in the surface of the first variable resistance layer 106a, and a crossing point of the steps 106ax1 and 106ax2 is formed at the central part of the element. The first variable resistance layer 106a is segmented into four regions, with the crossing point as a starting point. According to this configuration, the largest bend 106bx of the second variable resistance layer 106b is created above the crossing point of the steps 106ax1 and 106ax2 of the first variable resistance layer 106a, and thus it is possible to cause the break phenomenon even with a low voltage, with the bend 106bx as a starting point.